This invention relates to a radiographic image storage panel and a process for reading out a radiographic image information stored in the storage panel, and especially to a radiographic image storage panel and a process for reading out a radiographic image information stored in the storage panel which provides a radiographic image having a high sharpness.
Such a radiographic image as an X-ray image has been widely used for diagnosis of diseases and the like. For the purpose of obtaining such an X-ray image, an X-ray radiophotography wherein a phosphor layer (a fluorescent screen) is irradiated with X-rays which are transmitted by a subject to have visible rays produced, and a silver salt-coated photographic film is irradiated with the produced visible rays and then developed as in the common photographic way, is applied. In recent years, however, methods to take out a photographic image directly from the fluorescent screen without using a silver salt-coated film have come to be developed.
As such a method, there is cited, for example, a method wherein first, radioactive rays which are transmitted by a subject are made absorbed by a fluorescent screen, then said screen is made excited by light or thermal energy or the like so that said screen may emit as fluorescence radiation energy which is absorbed and accumulated thereby, and then said emitted fluorescence is detected to make an image. For example, methods of radiographic image storage to use a stimulable phosphor and to use visible or infrared rays as stimulable excitation rays have been proposed by U.S. Pat. No. 3,859,527 and Japanese Patent O.P.I. Publication No. 12144/1980. In a radiographic image storage of these methods, the layer containing a stimulable phosphor is formed on a support. This layer is irradiated with radioactive rays which are transmitted by the subject to accumulate radiation energy in accordance with transmission rate of every site of the subject so as to build a latent image, and then scanned with stimulable excitation rays to make accumulated radiation energy emit site by site. The emitted radiation energy is converted to light, and then the image is made according to optical signals based on variation of intensity of the light. This final image is allowed either to be regenerated as a hard copy, or to be regenerated on CRT.
As for the radiographic image storage panel, which has a layer containing a stimulable phosphor, of these methods, it is required that not only both the absorption rate of radiation and conversion rate to light (hereinafter both together are referred to as a whole "radiosensitivity") are high, but also the final image is good in graininess and sharpness, similarly to the case of afore-mentioned radiography using fluorescent screen.
However, since such a radiographic image storage panel, which has a layer containing a stimulable phosphor, is in general prepared by coating a dispersion which contains both stimulable phosphor particles with particle sizes ranging from 1 .mu.m to 30 .mu.m or so, and some organic binder, on its support or protective layer, the packing density of the stimulable phosphor is necessarily low as 50% for packing ratio, and consequently requires the layer containing the stimulable phosphor to be much thick to secure a sufficiently high radiosensitivity as shown in FIG. 3. While the quantity of the adhered stimulable phosphor is about 50 mg/cm.sup.2 when the thickness of the layer is about 200 .mu.m, the radiosensitivity linearly rises until the thickness reaches about 350 .mu.m, but levels off over about 450 .mu.m of thickness. Such a leveling-off of the radiosensitivity is caused by missing the emission of the stimulable fluorescence from the inside of the layer containing the stimulable phosphor due to the dispersing of the stimulable fluorescence among stimulable phosphor particles.
On the other hand, in the above radiographic image storage methods, the thicker the layer containing the stimulable phosphor is, the sharper the obtained image tends to be as shown in FIG. 4; therefore, it is necessary to make said layer thinner as possible for the requirement of improving the image sharpness.
In addition, since the image graininess according to the above radiographic image storage methods definitely depends on the locational fluctuations of number of radiation quantums (quantum mottles), and/or structural disturbances of the layer containing the stimulable phosphor of the radiographic image storage panel (structure mottles, the thinning of said layer causes the increase in quantum mottles through the decrease in number of radioactive quantums absorbed in said layer, and/or the decrease in structure mottles through the actualization of structural disturbances, resulting in the deterioration of the image quality. Therefore, it is necessary to make said layer thicker as possible for the requirement of improving of the image graininess.
Thus, so far as the thickness of the layer containing a stimulable phosphor in such a conventional radiographic image storage panel, both the radiosensitivity or graininess of the resulting image, and the sharpness of the image present quite opposite requirements; therefore said panel has been made necessarily at some compromise between different requirements of both the radiosensitivity or graininess, and the sharpness.
Whereas, as well known, the image sharpness in the conventional radiography depends on the diffusion of the instantaneous fluorescence (the fluorescence on irradiating) by the phosphor in the fluorescent screen, the image sharpness in the above radiographic image storage method using the stimulable phosphor does not depend on the diffusion of the stimulable fluorescence by the stimulable phosphor in the radiographic image storage panel, but depends on the extension of the stimulable excitation rays in said panel. The reason for this is as follows. That is, since, in such a radiographic image storage method, the radiographic image information accumulated in the panel is taken out thereof on a time series basis, the stimulable fluorescence by the stimulable excitation rays irradiated at a certain time (t.sub.i) is desirably recorded as the output from a certain pixel (x.sub.i, y.sub.i) on said panel which is irradiated with said stimulable excitation rays at that time (t.sub.i). If said stimulable excitation rays had extended in said panel due to dispersing or the like, and excited even stimulable phospor particles outside the pixel (x.sub.i, y.sub.i) actually irradiated, besides, the output from the area wider than said pixel (x.sub.i, y.sub.i) would be recorded as the output from said pixel. Therefore, if the stimulable fluorescence by the stimulable excitation rays irradiated at the time (t.sub.i) is the only fluorescence from said pixel (x.sub.i, y.sub.i) on the panel truly irradiated by said stimulable excitation rays at that time (t.sub.i), the sharpness of the obtained image is regardless of the diffusion of said fluorescence.
Under such situations some methods to improve the sharpness of the radiographic image have been proposed, including: a method of incorporating certain white powder into a layer containing a stimulable phosphor of a radiographic image storage panel by Japanese Patent O.P.I. Publication No. 146447/1980; and a method of coloring a radiographic image storage panel so that the mean reflecting rate of rays of the stimulable excitation wavelength region by the stimulable phosphor may be lower than that of rays of the stimulable fluorescent wavelength region by Japanese Patent O.P.I. Publication No. 163500/1980, for example. These methods however bring about the reduction of sensitivity inevitably in exchange of the improvement of image sharpness, and so are not evaluated as desirable.